1. Field of the Invention
This invention relates generally to the use of a zero-valent metal emulsion to remediate halogenated solvents found in water. Preferably, zero-valent iron emulsions containing nanoscale or microscale iron particles are used to remediate dense non-aqueous phase liquid (DNAPL) sources found in groundwater.
2. Description of the Background Art
Remediation of halogenated solvents, such as trichloroethylene (TCE), halogenated hydrocarbons, and other chlorinated solvents, is of great concern due to their toxicity and their persistence in the environment. Halogenated solvents, such as TCE, enter the groundwater and soil environments through improper disposal practices. These halogenated solvents are used in industry as degreasers; in the production of dry cleaning fluids, spot removers, insecticides and pesticides, as well as in many other manufacturing processes. Because of halogenated solvents' wide variety of uses they have become ubiquitous in the environment. According to the U.S. Environmental Protection Agency (USEPA), TCE has been found in at least 852 of the 1430 National Priorities List sites (ATSDR 1995).
When released into the ground, halogented solvents, such as TCE, will sink through the subsurface soil and groundwater until it is contained by a non-permeable surface such as bedrock. At this point it will pool and slowly dissolve into any water that it comes into contact with. Halogenated solvents, such as TCE, that have higher densities than water are referred to as dense nonaqueous phase liquids (DNAPLs). Due to the low solubility of many halogenated solvents, for example, TCE's low solubility (1.1×103 mg/L), the pool will continue to contaminate groundwater for extended periods of time. As the groundwater is in constant motion, this pool can contaminate very large areas of potential drinking water. Breakdown of the halogenated solvents in natural environments is very slow and produces other potential harmful by-products that are also regulated by the USEPA in Title 40 Code of Federal Regulations. Currently, the maximum contaminant level of TCE acceptable in ground water established by the EPA is 4–5 μg/L.
Traditionally, the method of choice for remediation of TCE has been accomplished by pumping the contaminated water to a surface plant and removing the TCE by air stripping or granular activated carbon adsorption. The decontaminated water is then disposed of into wastewater treatment plants or re-injected into the ground.
Pump-and-treat technology has many limitations. Installing the surface plant is very costly. Additionally, although the initial depletion of TCE is quite high, the depletion levels off to values that are sometimes above the regulatory levels. The surface plant requires constant monitoring; it produces hazardous wastes, and requires an energy source to operate the pumps and strippers. Due to TCE's low solubility in water, remediation of the groundwater using pump-and-treat technologies will take very long periods of time (e.g. decades) in order to maintain protection of human health and the environment. The pump and treat technologies primarily provide containment, rather than remediation. Because of the length of time necessary in the pump and treat technologies, high operation and maintenance cost over the time period of remediation are incurred.
Several pilot and full-scale projects for remediation of DNAPLs employ the use of a permeable reactive barrier wall (PRBW) placed within groundwater. The PRBW is installed across a path of a contaminated plume. The contaminants are removed or degraded producing decontaminated water on the down gradient side of the wall. The use of zero-valent metals, such as iron, to reductively dechlorinate DNAPLs has been employed as the reactive material in these PRBWs. The use of PRBWs has several advantages over the traditional pump-and-treat methods of remediation. This process produces little waste and is much less labor intensive. Since it is a passive system, mechanical failures are eliminated. The most prominent drawback of the use of an in-situ permeable reactive wall is that, like pump-and-treat systems, it never actually treats the contaminant pool. These processes rely on the DNAPL dissolution and transport for treatment. Again, the process of complete remediation will take an extended period of time.
Currently, there are no available proven technologies that can treat 100% of DNAPL sources. These sources include free-phase, residual phase, and sorbed (or matrix diffused) phases of DNAPL. Attempts have been made to remove the DNAPL sources through heating to enhance volatilization. Such heating techniques have included steam injection and radio-frequency-heating. However, this approach is limited because of the energy costs associated with heating the groundwater and the exponential volume of areas that will need to be treated to ensure that the entire DNAPL source is encountered and treated.
An alternative approach has been to flood the source area with surfactants or oxidizing agents. DNAPL contaminates are remediated by injecting a surfactant to either solubilize or mobilize the DNAPL pool. The presence of surfactant micelles increases the solubility of the DNAPL in the groundwater. This method of remediation is unique in that it actually confronts the pools of DNAPL. However, DNAPLs such as TCE are more subject to uncontrolled migration using this technique and could produce larger contamination zones. Additionally, these surfactants only travel through most permeable zones. DNAPL pools diffuse into geological areas of low permeability preventing their 100% removal that is required to prevent the remaining DNAPL from re-contaminating the groundwater.
Therefore, a critical need exists for technologies that can effectively treat DNAPL sources in the saturated zone and result in both their destruction and containment with reduced treatment times and lower costs.